In the present invention, we are concerned with axial flux permanent magnet machines. Broadly speaking these have disc- or ring-shaped rotor and stator structures arranged about an axis. Typically the stator comprises a set of coils each parallel to the axis and the rotor bears a set of permanent magnets and is mounted on a bearing so that it can rotate about the axis driven by fields from the stator coils. FIG. 1a shows the general configuration of an axial flux machine of the present invention with a pair of rotors R1, R2 to either side of a stator S—although a simple structure of the present invention could omit one of the rotors. As can be seen there is an air gap G between a rotor and a stator and in an axial flux machine the direction of flux through the air gap is substantially axial.
There are various configurations of axial flux permanent magnet machine depending upon the arrangement of north and south poles on the rotors. FIG. 1b illustrates the basic configurations of a Torus NS machine, a Torus NN machine (which has a thicker yoke because the NN pole arrangement requires flux to flow through the thickness of the yoke), and a YASA (Yokeless and Segmented Armature) topology. The illustration of the YASA, so called “Y” machine topology shows cross-sections through two coils, the cross-hatched area showing the windings around each coil. As can be appreciated, dispensing with the stator yoke provides a substantial saving in weight and iron losses, but impacts on the ability to extract heat from the machine.
One difficulty with electric machines generally is to provide adequate cooling. This is a particular problem with a Y machine having a high torque density because significant heat is generated in the coils at high torques and is often a limiting factor in the torques that can be employed, at least for extended periods of time. The problem with cooling the stator in such a machine was addressed in our previous application GB2468018. However, with axial flux machines, eddy currents in the magnets and rotor structure (housing the magnets) may cause the rotor and the magnets to heat up. Whilst cooling the stator also cools the rotor to some extent (through heat being transferred by a hub or bearing assembly, for example, between the rotor and stator), there still may remain a significant amount of heat in the rotors, which will adversely affect the performance of the machine, since the properties of the magnets deteriorate as their temperature increases.
Some solutions have been proposed in which air channels are created in the surface of the magnets, between magnets, or blades are placed at the ends of rotors in order to move air around the magnets and rotor for air cooling the rotor. In some machines, fan blades are placed on the rotor in order to move air around when the rotor is turning. Examples include JP2009022146, EP2843812 and DE4214483. However, often these solutions impact on the design of the machine, for example, by increasing the axial length of the machine, which is not desirable. Furthermore, due to tolerances in the design and manufacture of the machines, the cooling performance of such fans and structures varies from machine to machine, which is undesirable. For some applications radial motors are cooled by way of a fan attached to the motor shaft and which because of the radial structure is able to pass air over the gap between stator and rotor and thereby cool both components. Such a configuration is not feasible with an axial flux motor.
We have therefore appreciated the need for an improved axial flux machine in which the rotor is cooled more reliably.